The LC displays used at present are mostly those of the TN (twisted nematic) type. However, these have the disadvantage of a strong viewing-angle dependence of the contrast.
In addition, so-called VA (vertical alignment) displays are known which have a broader viewing angle. The LC cell of a VA display contains a layer of an LC medium between two transparent electrodes, where the LC medium usually has a negative value of the dielectric (DC) anisotropy. In the switched-off state, the molecules of the LC layer are aligned perpendicular to the electrode surfaces (homeotropically) or have a tilted homeotropic alignment. On application of an electrical voltage to the electrodes, a realignment of the LC molecules parallel to the electrode surfaces takes place.
Furthermore, OCB (optically compensated bend) displays are known which are based on a birefringence effect and have an LC layer with a so-called “bend” alignment and usually positive (DC) anisotropy. On application of an electrical voltage, a realignment of the LC molecules perpendicular to the electrode surfaces takes place. In addition, OCB displays normally contain one or more birefringent optical retardation films in order to prevent undesired transparency to light of the bend cell in the dark state. OCB displays have a broader viewing angle and shorter response times compared with TN displays.
Also known are IPS (in-plane switching) displays, which contain an LC layer between two substrates, but wherein the two electrodes are located only on one of the substrates, usually with comb-shaped, interdigital structures. When applying a voltage to the electrodes, an electric field which has a significant component parallel to the LC layer is thereby generated. This causes realignment of the LC molecules in the layer plane. Furthermore, so-called FFS (fringe field switching) displays have been proposed (see, inter alia, S. H. Jung et al., Jpn. J. Appl. Phys., Volume 43, No. 3, 2004, 1028), which likewise contain two electrodes on the same substrate, but, in contrast to IPS displays, only one of these is in the form of a structured (comb-shaped) electrode, and the other electrode is unstructured. A strong, so-called “fringe field” is thereby generated, i.e. a strong electric field close to the edge of the electrodes, and, throughout the cell, an electric field which has both a strong vertical component and a strong horizontal component. Both IPS displays and also FFS displays have a low viewing-angle dependence of the contrast.
In VA displays of the more recent type, uniform alignment of the LC molecules is restricted to a plurality of relatively small domains within the LC cell. Disclinations can exist between these domains, also known as tilt domains. VA displays having tilt domains have, compared with conventional VA displays, a greater viewing-angle independence of the contrast and the grey shades. In addition, displays of this type are simpler to produce since additional treatment of the electrode surface for uniform alignment of the molecules in the switched-on state, such as, for example, by rubbing, is no longer necessary. Instead, the preferential direction of the tilt or pretilt angle is controlled by a special design of the electrodes. In so-called MVA (multidomain vertical alignment) displays, this is usually achieved by the electrodes having protrusions which cause a local pretilt. As a consequence, the LC molecules are aligned parallel to the electrode surfaces in different directions in different, defined regions of the cell on application of a voltage. “Controlled” switching is thereby achieved, and the formation of interfering disinclination lines is prevented. Although this arrangement improves the viewing angle of the display, it results, however, in a reduction in its transparency to light. A further development of MVA uses protrusions on only one electrode side, while the opposite electrode has slits, which improves the transparency to light. The slitted electrodes generate an inhomogeneous electric field in the LC cell on application of a voltage, meaning that controlled switching is still achieved. For further improvement of the transparency to light, the separations between the slits and protrusions can be increased, but this in turn results in a lengthening of the response times. In the so-called PVA (patterned VA), protrusions are rendered completely superfluous in that both electrodes are structured by means of slits on the opposite sides, which results in increased contrast and improved transparency to light, but is technologically difficult and makes the display more sensitive to mechanical influences (tapping, etc.). For many applications, such as, for example, monitors and especially TV screens, however, a shortening of the response times and an improvement in the contrast and luminance (transmission) of the display are desired.
A further development are the so-called PS (polymer sustained) or PSA (polymer sustained alignment) displays, also known as “polymer stabilised” displays. In these, a small amount (for example 0.3% by weight, typically <1% by weight) of a polymerisable compound is added to the LC medium and, after introduction into the LC cell, is polymerised or crosslinked in situ, usually by UV photopolymerisation, optionally with an electrical voltage applied between the electrodes. The addition of polymerisable mesogenic or liquid-crystalline compounds, also known as “reactive mesogens” (RMs), to the LC mixture has proven particularly suitable.
In the meantime, the PS or PSA principle is being used in diverse classical LC displays. Thus, for example, PSA-VA, PSA-OCB, PS-IPS and PS-TN displays are known. In PSA-VA and PSA-OCB displays polymerisation is usually carried out while a voltage is applied to the electrodes, whereas in PSA-IPS displays polymerisation it is carried out with or without, preferably without application of a voltage. As can be demonstrated in test cells, the PSA method results in a pretilt in the cell. In the case of PSA-OCB displays, it is therefore possible for the bend structure to be stabilised so that an offset voltage is unnecessary or can be reduced. In the case of PSA-VA displays, this pretilt has a positive effect on response times. For PSA-VA displays, a standard MVA or PVA pixel and electrode layout can be used. In addition, however, it is possible, for example, to manage with only one structured electrode side and no protrusions, which significantly simplifies production and at the same time results in very good contrast at the same time as very good transparency to light.
PSA-VA displays are described, for example, in JP 10-036847 A, EP 1 170 626 A2, U.S. Pat. No. 6,861,107, U.S. Pat. No. 7,169,449, US 2004/0191428 A1, US 2006/0066793 A1 and US 2006/0103804 A1. PSA-OCB displays are described, for example, in T.-J-Chen et al., Jpn. J. Appl. Phys. 45, 2006, 2702-2704 and S. H. Kim, L.-C-Chien, Jpn. J. Appl. Phys. 43, 2004, 7643-7647. PS-IPS displays are described, for example, in U.S. Pat. No. 6,177,972 and Appl. Phys. Lett. 1999, 75(21), 3264. PS-TN displays are described, for example, in Optics Express 2004, 12(7), 1221.
PSA displays, like the conventional displays described above, can be operated either as active matrix or passive matrix displays. In active matrix type displays the individual pixels are usually addressed by integrated, non-linear active elements like for example thin film transistors (TFT), in passive matrix type displays by multiplexing, with both methods being well-known from prior art.
In particular for monitor and especially TV applications, optimisation of the response times, but also of the contrast and luminance (i.e. also transmission) of the LC display, is still demanded. The PSA process still appears to provide crucial advantages here. In particular in the case of PSA-VA, a shortening of the response times, which correlate with a measurable pretilt in test cells, can be achieved without a significant adverse effects on other parameters.
However, it has been found that the LC mixtures and RMs known from the prior art still have some disadvantages on use in PSA displays. Thus, not every desired soluble RM is also suitable for PSA displays, and it often appears difficult to find more suitable selection criteria than the direct PSA experiment with pretilt measurements. The choice becomes even smaller if polymerisation by means of UV light without the addition of photoinitiators is desired, which may be advantageous for certain applications.
In addition, the selected material system of LC mixture (also referred to as “LC host mixture”) and polymerisable component should have the best possible electrical properties, in particular a high “voltage holding ratio” (HR or VHR). A high HR after irradiation with UV light is important especially for use in a PSA display, because UV irradiation is an indispensable part of its manufacturing process, although it can also occur as “normal” stress in the finished display.
However, the problem arises that not every combination of LC mixture and polymerisable component works is suitable for use in PSA displays since, for example, an inadequate tilt or no tilt at all is established or since, for example, the HR is inadequate for TFT display applications.
In particular it is desired to have available novel and improved materials for PSA displays which enable the generation of a small pretilt. Especially desired are materials which will, during polymerisation, either generate a smaller pretilt after the same UV irradiation time as used for prior art materials, and/or generate the same pretilt as the prior art materials already after shorter exposure time. This allows to reduce the manufacturing time (tact time) and the manufacturing costs for the display.
Another problem when manufacturing PSA displays ist the presence and removal of unreacted RMs after the polymerisation step used for tilt angle generation. Such unreacted RMs can negatively affect the display properties and performance, for example by uncontrolled polymerisation in the display during its operation.
Thus, PSA displays of prior art often show the undesired “image sticking” or “image burn” effect, wherein the image generated in the display by addressing selected pixels remains visible, even when the voltage for this pixel has been switched off, or when other pixels have been addressed.
Image sticking can occur for example when using LC host mixtures with a low HR, wherein the UV component of ambient light or emitted by the display backlight can induce undesired cleavage reactions in the LC molecules. This can lead to ionic impurities which are enriched at the electrodes or alignment layers, where they cause a reduction of the effective voltage applied to the display. This effect is also known for conventional displays not containing a polymeric component.
In PSA displays an additional image sticking effect can be observed which is caused by the presence of residual unpolymerised RMs. In such displays the UV component of ambient light or emitted by the backlight causes undesired spontaneous polymerisation of the unreacted RMs. In the addressed pixels this can change the tilt angle after several addressing cycles, thereby causing a change of the transmission, whereas in the unaddressed pixels the tilt angle and transmission remain unaffected.
It is therefore desirable that the polymerisation reaction when manufacturing the PSA display is as complete as possible, and that the amount of residual unpolymerised RMs in the PSA display after its manufacture is as low as possible.
For these purposes RMs and LC host mixtures are desired which enable a complete and effective polymerisation reaction. In addition it is desired to achieve a controlled polymerisation of any residual amounts of unreacted RMs that are still present in the display. Also, RMs and LC host mixtures are desired that enable a faster and more effective polymerisation than the materials currently known.
Another problem is that conventional RMs used for manufacturing PSA displays by UV photopolymerisation often show maximum UV absorption at short wavelengths, especially below 300 nm. However, in the manufacturing process of PSA displays it is desired to avoid exposure to UV radiation of such short wavelengths, because these “hard UV components” are hazardous and increase the risk of damaging the various materials and components used in the display. Therefore, display manufacturers preferably use UV exposure systems of longer wavelengths, especially over 320 nm or even over 350 nm.
It is therefore desired to have available materials and material combinations, especially RMs and LC host mixtures, for use in PS or PSA displays, which are suitable to solve the above-mentioned problems. In particular, the materials should provide one or more of the following improvements:                enable effective polymerisation using longer UV wavelengths, especially of 320 nm or more, preferably 350 nm or more,        provide better protection against negative influence of the UV irradiation used for photopolymerisation of the RMs,        generally provide improved UV stability,        allow a faster and more effective polymerisation reaction,        reduce the amount of residual unpolymerised RMs in the display,        enable the faster generation of small tilt angles and/or the generation of smaller tilt angles compared to PSA displays and materials of prior art,        reduce the image sticking in the PSA display.        
It was an aim of the present invention to provide novel PSA displays and novel materials for use in PSA displays, in particular LC host mixtures and RMs, which are suitable for solving the above-mentioned problems, do not have the disadvantages described above, or only do so to a smaller extent, and provide one or more of the above-mentioned improvements and advantages.
In addition, the PSA displays should have high specific resistance at the same time as a large working-temperature range, short response times, even at low temperatures, and a low threshold voltage, which facilitate a large number of grey shades, high contrast and a wide viewing angle, and have high values for the HR after UV exposure. In PSA displays for mobile applications, the LC media should show low threshold voltage and high birefringence.
Surprisingly, it has now been found that these objects can be achieved by using LC compounds, LC mixtures and LC media according to the present invention as described hereinafter in PSA displays. In particular, it has surprisingly been found that, when using LC host mixtures containing specific terphenyl or quaterphenyl compounds in combination with RMs, it is possible to polymerise the RMS at higher wavelength and with UV radiation dosage, provide improved protection against hazardous and damaging UV light, enable photopolymerization with longer UV wavelengths, and achieve photopolymerisation of the RMs that is faster, more effective and more complete, compared to LC host mixtures of prior art. Also, it enables a faster generation of the pretilt angle and a reduction of the UV exposure time and/or UV intensity and/or UV radiation dose, allowing a more time- and cost-effective manufacturing process. Also, it allows to reduce the residual amount of unreacted RMs and to suppress the image sticking effect.